In the power system with multiple harmonic sources, there is interaction and influence between harmonic sources. The study of coupling mechanism and the calculation of coupling weight is very important in harmonic responsibility and harmonic decoupling. The multiple harmonic sources system is divided into concentrated and distributed system. For concentrated system, at high frequency, Thevenin equivalent circuit is proposed, and Norton equivalent circuit is given too. Then, Partial Least Squares(PLS) is used to estimate coupling index. For distributed system, estimate mutual impedance and provide coupling index. Finally, the MATLAB model is built in concentrated system with 5 harmonic sources; the distributed system utilize IEEE 13 bus system as simulation model to calculate coupling weight.